World energy supplies are substantially impacted by the world's heavy oil resources. Indeed, heavy oil comprises 2,100 billion barrels of the world's total oil reserves. Processes for the economic recovery of these viscous reserves are clearly important.
Asphalt, tar, and heavy oil are typically deposited near the surface with overburden depths that span a few feet to a few thousands of feet. In Canada, vast deposits of heavy oil are found in the Athabasca, Cold Lake, Celtic, Lloydminster and McMurray reservoirs. In California, heavy oil is found in the South Belridge, Midway Sunset, Kern River and other reservoirs.
In large Athabasca and Cold Lake bitumen deposits oil is essentially immobile--unable to flow under normal natural drive primary recovery mechanisms. Furthermore, oil saturations in these formations are typically large. This limits the injectivity of a fluid (heated or cold) into the formation. Moreover, many of these deposits are too deep below the surface to be mined effectively and economically.
In-situ techniques of recovering viscous oil and bitumen have been the subject of much previous investigation. These techniques can be split into three categories: 1) cyclic processes involving injecting and producing a viscosity reducing agent; 2) continuous steaming processes which involve injecting a heated fluid at one well and displacing oil to another set of wells; and 3) the relatively new Steam (or Solvent) Assisted Gravity Drainage process.
Each of these techniques has large limitations if application to the very viscous Athabasca or Cold Lake reservoirs is desired.
Cyclic steam or solvent stimulation in these two reservoirs are severely hampered by the lack of any significant steam injectivity into the respective formations. Hence, in the case of vertical wells a formation fracture is required to obtain any significant injectivity into the formation. Some success with a fracturing technique has been obtained in the Cold Lake reservoir at locations not having any significant underlying water aquifer. However, if a water aquifer exists beneath the vertical well located in the oil bearing formation, fracturing during steam injection results in early and large water influx during the production phase. This substantially lowers the economic performance of wells. In addition, cyclic steaming techniques are not continuous in nature thereby reducing the economic viability of the process. Clearly, steam stimulation techniques in Cold Lake and Athabasca are severely limited.
Vertical well continuous steaming processes are not technically or economically feasible in the very viscous bitumen reservoirs. Oil mobility is simply far too small to be produced from a cold production well as is done in California type of reservoirs. Steam injection from one well and production from a remote production well is not possible unless a formation fracture is again formed. Formation fractures between wells are very difficult to control and there are operational problems associated with fracturing in such a controlled manner as to intersect an entire pattern of wells. Hence, classical steam flooding, even in the presence of initial fluid injectivity artificially induced by a fracture has significant limitations.
Steam Assisted Gravity Drainage (SAGD) is disclosed in U.S. Pat. No. 4,344,485 which issued to Butler in 1982. SAGD uses a pair of horizontal wells connected by a vertical fracture. The process has several advantages to steam stimulation or continuous steam injection. One advantage is that initial steam injectivity is not needed as steam rises by gravity above the upper well thereby replacing oil produced at the lower well. Another advantage is that since the process is gravity dominated and steam replaces voided oil, good sweep efficiency is obtained. Yet another advantage is since horizontal wells are utilized, good oil rates may be obtained by simply extending the length of the well to contact more of the oil bearing formation. In the SAGD process, steam is injected in the upper horizontal well while oil and water are produced at the lower horizontal well. Steam production from the lower well is controlled so that the entire process remains in the gravity dominated regime. A steam chamber rises above the upper well and oil warmed by conduction drains along the outside of the chamber to the lower production well. The process has the advantages of high oil rates and good overall recovery. It can be used in the absence of a vertical fracture.
However, one serious limitation of this process in practical application is the need to have two parallel horizontal wells--one beneath the other. Those skilled in the art of drilling horizontal wells will immediately recognize the difficulty in drilling two parallel horizontal wells, one above the other, with any real accuracy for any real horizontal distance from the surface.
Thus, what is needed is a process that provides the advantages of the Steam Assisted Gravity Drainage process but removes the difficulty of drilling two precisely spaced, parallel horizontal wellbores from the surface.